1. Field of the Invention
This present invention relates to a system for measuring resonance frequencies in turbine and compressor vanes and in propeller blades.
2. Description of the Prior Art
In an engine, the rotating blades of the rotor undergo excitations in a very wide range of frequencies. Because of this, the blades may go into resonance, and the amplitude of oscillation may reach destructive values. Knowing the exact value of these resonance frequencies in the various modes makes it possible to minimize the harmful effects by applying appropriate remedies.
According to a known process, the blade is attached rigidly by its seating and is excited at its base by means of a sound generator, in particular a speaker. The blade's resonance is detected at its tip by means of a displacement transducer (in particular a magnetic or optical transducer). This transducer furnishes the frequency and amplitude of the blade's resonance. The maximum is sought by varying the frequency of the excitation signal and by correlatively measuring its amplitude in the displacement pick-up. The drawback of such a method lies in its relative slowness and also in its poor precision. Indeed, in the vicinity of the resonance frequency, the amplitude curve varies very little, and finding the maximum proves to be a delicate procedure if good precision is desired.
Furthermore, it is known that the mechanical or electrical oscillation of a body is characterized by two curves, which are shown in FIGS. 1a and 1b. In FIG. 1a the curve of the amplitude of A has been represented as a function of the frequency F which is utilized in the aforementioned procedure. In FIG. 1b the curve of the variation between the vibration (electrical or mechanical) phase of the excited body and the phase of the exciter signal taken in the plane of excitation is shown. As is visible in FIG. 1b, this curve has the peculiarity of passing through zero at the resonance frequency, and this is furthermore the place at which its slope is most accentuated.